So-called recording measuring instruments or recorders are employed for graphic recording of a dependent variable as a function of an independent variable. Numerous types of such recorders have become known. A type frequently used particularly in electronics and electro-acoustics is the so-called level and frequency response recorder which permits recording of electrical and/or electro-acoustical values (levels) as functions of time and/or frequency. In the first case, there is obtained a so-called time-level diagram, for example, the variation of a sound level as a function of time; in the second case, a so-called frequency-characteristic (frequency response), as, for example, the variation with frequency of the sound pressure emitted by a loudspeaker. The recording of the tested value, the dependent variable, should be possible both on a linear as well as on a logarithmic scale, for example, in decibels (db), as is necessary, for example, in electro-acoustics because of the wide, required range of representation (1:1000). The X-axis in frequency response recorders is divided on a logarithmic scale also because three decade ranges of frequency (20 Hz to 20 kHz) must be recorded.
Level recorders known so far include, inter alia, a motor-operated paper feeding device and means for varying the feeding rate, also a usually electrodynamic device, based on the moving coil principle for linearily moving the recording stylus, and an electromechanical servo-system for achieving the desired logarithmic indication, and electric test amplifiers with rectifiers. Time diagrams can be recorded by means of such devices. When frequency characteristics are to be determined, there is additionally required a frequency generator having a mechanically adjustable, frequency-setting control element which is operated by the motor of the recorder synchronously with the paper feed by suitable means, for example, a flexible shaft, so that the corresponding generator frequency is associated with each position of the recording paper provided with frequency indicia. Because of the linear drive system employed and the servo control, these known test devices are very large and very expensive and were built heretofore only as non-portable laboratory testing devices. Recorder and audio frequency generator are separate test device units. Because of the servo control (control loop) and the servo potentiometers employed, these known testing devices have a relatively great tendency to instability, particularly at high recording rates and also low resolution. Only slow rates of paper movement and corresponding rates of frequency change are possible, therefore, with rapidly changing test signals. Since the servo-potentiometers are not of a continuous construction but are variable in steps only, their resolution is limted. The time constant of the control loop must be adapted to the time constant of the rectifier and to the rate of change of the signals. Otherwise great indication errors because of excessive or insufficient damping of the indication must be accepted, that is, the recording of the peaks and troughs in the frequency response curve is excessively high or unbalanced. The lower limiting frequency of the control loop also had to be adjusted according to the signal in order to avoid control oscillations at low frequencies. It is an additional shortcoming of these known devices that the synchronization of the audio frequency generator and recorder is time consuming and a change in the recording scale relatively complicated because it can be achieved only by replacing the very expensive measuring potentiometers. The measuring potentiometers are subject to mechanical wear whereby the accuracy of the indication is adversely affected.